The present invention relates to a magnetic recording medium which is excellent in running durability and running properties and shows high electromagnetic properties.
With demands for long time running, compact size, high density and high performance, magnetic recording media such as audio tapes and video tapes have been required to realize high electromagnetic properties. For example, according as video tape recorders are widely used, magnetic recording media of much higher running durability are desired, because it is thought that magnetic heads of the video tape recorders available in the beginning are too much abraded owing to repeated uses for a long period of time and troubles relating to the running durability such as head clogging easily take place.
According as the surface of the magnetic layer of the magnetic recording medium is made smoother, the electromagnetic properties of the medium becomes higher, but the friction coefficient of the magnetic layer in the running procedure becomes larger, and the running durability is lowered.
It is known that an abrasive having a Mohs hardness of not less than 6 is included in the magnetic layer, as described in Japanese Patent Publication No. 21048/1990 (Japanese Patent Laid-Open Publication No. 94522/1981) and Japanese Patent Publication No. 53687/1991 (Japanese Patent Laid-Open Publication No. 179945/1982). If the particle size of the abrasive to be included in the magnetic layer is enlarged, the durability of the magnetic recording medium is generally improved, but on the other side, surface properties of the magnetic layer are deteriorated thereby to lower the electromagnetic properties.
Under the circumstances, there has been proposed a magnetic recording medium having such a constitution that the magnetic layer consists of two layers of an upper layer and a lower layer, an abrasive having a Mohs hardness of not less than 6 is contained in only the upper layer, and the mean particle diameter of the abrasive is not larger than the thickness of the upper layer, as described in Japanese Patent Laid-Open Publication No. 106331/1989.
In this magnetic recording medium, lowering of the electromagnetic properties caused by adding the abrasive having a Mohs hardness of not less than 6 to the magnetic layer is prevented to a certain degree, but the running durability is still insufficient, and especially when a video tape recorder is used for a long period of time as mentioned above, troubles relating to the running durability easily take place.